The present invention is directed at a process for preparing a K-type polarizer characterized by a uniaxially oriented film of poly(vinylalcohol) having light polarizing (dichroic) blocks of conjugated poly(acetylene).
Dichroic polarizers are absorptive, linear polarizers having a vectoral anisotropy in the absorption of incident light. The polarizer, therefore, has the property of differential absorption (and transmission) of the components of an incident beam of light depending on the direction of vibration of the components. Generally, the polarizer will transmit radiant energy along one electromagnetic vector and absorb energy along a perpendicular electromagnetic vector. A beam of incident light, on entering the dichroic polarizer, encounters two different absorption coefficients, one low and one high so that the emergent light vibrates substantially in the direction of low absorption (high transmission).
The development of synthetic polarizers has made possible the widespread utility of light-polarizing elements for a wide variety of applications, such as in liquid crystal display screens in which crossed polarizers are used in conjunction with an addressable liquid crystal material to provide the basis for image formation. Polarizers have also been used in many optical applications, such as to reducing glare or the brightness of specular reflection in photography or CRT monitors to reduce glare.
Among the known synthetic polarizers are xe2x80x9cK-typexe2x80x9d polarizers in which the linear dichroic light polarizing materials are prepared by dehydration of poly(vinyl alcohol). K-type polarizers may also be known as inherent polarizers since the absorbing chromophore is the result of conjugation in the polymer backbone, rather than due to dyes added to the polymer matrix. These polarizers comprise a sheet of oriented poly(vinyl alcohol) having light polarizing (dichroic) molecules of poly(acetylene) blocks (i.e. xe2x80x94[CHxe2x95x90CHxe2x80x94]n formed by heating the oriented poly(vinyl alcohol) sheet in the presence of a dehydration catalyst such as vapors of aqueous hydrochloric acid. By orienting the poly(vinyl alcohol) matrix uniaxially the transition moments of the chromophores, the conjugated poly(acetylene) blocks, are also oriented and the material becomes visibly dichroic.
While K-type polarizers can be made by conventional acid processes, these processes necessarily involving the handling of, and potential exposure to, hazardous quantities of acid, usually hydrochloric acid. Additionally, the vapor-phase acid processes can result in non-uniform catalytic dehydration, which can lead to streaking or mottling of the polarizer, rendering it unsuitable for many precision optical applications. See, for example U.S. Pat. No. 5,973,834 (Kadaba et al.). Hence, there is a need for a process for preparing K-type polarizers that does not use large quantities or hazardous and corrosive acids (such as HCl vapors) to effect dehydration and can produce high quality, uniform polarizers.
The present invention provides a process for preparing a polarizer whereby an article comprising an oriented, vinylalcohol polymer film layer, and an acid donor layer comprising a photoacid generator, is exposed to radiant energy at a temperature sufficient to effect partial dehydration of the vinylalcohol polymer to a vinylalcohol/poly(acetylene) copolymer. When exposed to radiant energy, typically ultraviolet light, the photoacid generator reacts photochemically to release one or more molecules of acid. The incipient acid then reacts catalytically with the vinylalcohol polymer to dehydrate it, producing a vinylene segment (i.e. xe2x80x94CHxe2x95x90CHxe2x80x94, which may also be referred to as poly(acetylene) blocks) along the chain of the vinylalcohol polymer. As the reaction proceeds, these vinylene segments grow in number and produce varying lengths of conjugated vinylene segments, the segments being relatively uniformly distributed in the polymer matrix. For example, the polymer resulting from partially dehydrated poly(vinylalcohol) may have the general structure: 
where xe2x80x94(CH2xe2x80x94CHOHxe2x80x94)axe2x80x94 represent blocks of poly(vinyl alcohol), xe2x80x94(CHxe2x95x90CH)bxe2x80x94 represents conjugated blocks of poly(acetylene), a and b are numbers such that a+b is at least 500, preferably at least 1000, a greater than b, and b is sufficiently large to produce a conjugated chromophore. Generally b is about 2 to 30. It will be understood that a particular polymer chain may comprise more than one of the above blocks. The conjugated blocks of xe2x80x94(CHxe2x95x90CH)bxe2x80x94 may also be referred to as vinylene blocks or poly(acetylene) herein.
The orientation of the polymer chains in combination with the concentration of the blocks of conjugated vinylene blocks imparts a dichoism to the film layer. Concurrent with, or subsequent to, the photochemical reaction, the article may be heated at a temperature, and for a time, sufficient to effect the desired degree of dehydration and concurrent production of conjugated vinylene blocks (poly(acetylene) blocks).
The present invention provides a pre-polarizer article comprising an oriented vinyl alcohol polymer layer and an acid donor layer. The photoacid generator is dissolved or dispersed in the donor layer and upon irradiation the incipient acid diffuses into the adjacent vinylalcohol polymer matrix to effect partial dehydration of the vinylalcohol polymer to conjugated vinylene [poly(acetylene)] segments. As used herein xe2x80x9cpre-polarizerxe2x80x9d refers to an article having the aforementioned construction, and which, upon irradiation and heating, is converted to a K-type polarizer. The pre-polarizer article may further include a support layer for providing mechanical strength to the vinylalcohol polymer layer. The pre-polarizer article may further include a barrier layer for directing the diffusion of the incipient acid molecules and/or reducing loss of the acid from the exposed surfaces, and/or improving moisture resistance. The pre-polarizer article may further include an adhesive layer for securing the pre-polarizer, or the subsequently generated polarizer to a substrate. Advantageously the pre-polarizer allows one to produce custom polarizers with specified patterns or indicia, or with custom optical properties on an as-needed basis.
The method of the present invention overcomes deficiencies of the prior art by avoiding the use of large quantities of corrosive acid, whether in baths, in fuming processes, or as coatings in the processing steps. The use of a photoacid generator allows one to reduce the amount of acid necessary to effect the desired dehydration (relative to prior art), reduces potential hazardous exposure to the acid, while the production of the photo-generated acid can easily be controlled by control of the incident light. The method advantageously can produce high quality, uniform polarizers using conventional processing equipment and readily available polymers and photoacid generators. Further, the method may be used to produce polarizers bearing preselected patterns.